A mixed colour document is a document containing both colour and black and white information. Solely for the purpose of this disclosure, colour means all colours excluding black and all shades of grey. The simplest solution to reproduce a mixed colour document is to reproduce it entirely on a colour image reproduction system such as a colour printing and/or copying system. This will also assure the highest level of quality. However, colour image reproduction systems are known to have a higher cost per printed page and/or a lower productivity compared to a black and white image reproduction system. Therefore, particularly in a professional environment such as e.g. the repro department of a company or a print and/or copy shop, where multiple image reproduction systems are available, including a colour image reproduction system and a black and white image reproduction system and/or a colour image reproduction system capable of operating in a “black only” mode, it is desirable to make optimal use of the resources in order to optimize throughput and costs.
Image processing systems are known, as for instance disclosed in European Patent Application Publication EP1098243, and are able to split mixed colour documents into two parts. One part contains all the pages of the mixed colour document with colour information. The other part contains all the pages of the mixed colour document with black and white information. Subsequently, the respective parts are processed by a colour image reproduction system and a black and white image reproduction system respectively. Although this approach may already result in an improvement with respect to throughput and cost, the sole criterion to judge whether or not a page is to be classified as a colour page is the bare presence of some colour information.
More advanced algorithms could be used to provide the operator and/or user with the possibility to define a threshold in the colour content of a page, i.e. the amount of colour information present on the page. Below this threshold, such a page is treated as a black and white page, even although it contains a limited amount of colour information, e.g. a coloured line or bullet. Although this solution may have some merit, a disadvantage of using a colour threshold is that it does not take the nature of the colour information into account nor the user and/or operator intelligence, nor the format and/or purpose of the finally printed reproduction. For instance, the sole colour information of a page may be a coloured line in a graph. In such case it is quite possible that the amount of colour is below the predetermined colour threshold. Consequently, during processing that page is directed towards a black and white printer which may result in an unacceptable loss of information as it is quite possible that the graph can no longer be interpreted by the reader of the reproduced document.
On the other hand, if the sole colour information is some coloured text instead of a coloured line, the loss of information corresponding to the loss of colour could probably be acceptable, even when the amount of colour would be above the colour threshold.